Update bitnami/postgresql-repmgr Docker tag to v15.3.0

This MR contains the following updates:

Package	Type	Update	Change
bitnami/postgresql-repmgr	ironbank-docker	minor	15.2.0 -> 15.3.0
bitnami/postgresql-repmgr		minor	15.2.0 -> 15.3.0
bitnami/postgresql-repmgr	stage	minor	15.2.0 -> 15.3.0

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